1. Field of the Invention
The present invention relates to a lens inspection method and apparatus for judging whether the focal point of a lens is in a proper range, and more particularly to a lens inspection method and apparatus which is suitable for inspecting taking lenses of lens-fitted photographic film units during the manufacture of the film units.
2. Related Art
The focal length and the alignment of the optical axis may be considered the most fundamental factors which affect the performance of the taking lens of a camera. In compact cameras and lens-fitted photographic film units, the taking lenses are mounted in main housings. Therefore, if the taking lens is mounted off-center, or if the spacing between lens elements of the taking lens is incorrect, the optical axis or the focal point will deviate from normal positions, which degrades the optical properties of the taking lens. Other lenses, such as interchangeable lenses for single-reflex cameras, may also suffer the same trouble as above when mounted in lens barrels.
For this reason, these properties of lenses are inspected during or after the manufacture of the camera. In conventional lens inspections, for example, modulation transfer function (MTF)is measured, as is disclosed in JPA 55-20420 and JPA 58-118943, or an aerial image of a test chart is observed through a microscope. Alternatively, the image of a test chart is picked up as an image signal, and the image signal is analyzed.
However, because a large scale apparatus and a long time are necessary for MTF measurement, the lens inspection based on the MTF is unsuitable for mass-produced lenses.
Microscopic observation of the test chart image is basically a visual inspection, and so the result of this inspection tends to have variations in accuracy; the inspection is quite time-consuming.
Picking up the test chart image by a CCD camera and analyzing the image signal in a computer also requires a long time. Therefore, the third method is also impractical for inspecting mass-produced lenses efficiently.